LONDON (Reuters) - Britain's security services faced questions on Friday over whether they could have done more to prevent the murder of a soldier hacked to death in a busy London street after it emerged that his suspected killers were known to intelligence officers.
Suspects Michael Adebolajo, 28 and Michael Adebowale, 22, are under guard in hospital after being shot and arrested by police after the murder of 25-year-old Afghan war veteran Lee Rigby on Wednesday. They have not yet been charged.
Adebolajo, filmed justifying the killing as he stood near the body holding a knife and meat cleaver in bloodied hands, was born in Britain to a Nigerian immigrant family. Adebowale is a naturalised British citizen born in Nigeria.
Another man and a woman have also been arrested on suspicion of conspiracy to commit murder, an early indication that police are investigating whether the attack was part of a wider plot.
Prime Minister David Cameron said a parliamentary committee would carry out an investigation into the role of the security services. Britain's MI5 domestic spy agency had been aware of the men, but neither was considered a threat, a government source told Reuters.
Dramatic video footage showing the moment when police shot the two men was published on a British newspaper's website on Friday. The shaky, 10-second clip shows one of the men sprinting towards a police car with a knife in his hand before he is shot and tumbles to the ground.
"It is important for the public to know that the security services and the police are operating properly," former London police chief Ian Blair told BBC radio.
In an emotional news conference, Rigby's family said their "hearts have been ripped apart".
"You don't expect it to happen when he's in the UK. You think they're safe," said his tearful widow Rebecca Rigby, mother of their two-year-old son.
The attack has been condemned by mainstream British Muslim groups. It will increase attention on radical organisations like Al Muhajiroun, which organises provocative demonstrations against British troops and was banned in 2010.
Adebolajo, who converted to Islam and took the name "Mujahid" - warrior - attended lectures by Al Muhajiroun's Syrian-born founder Omar Bakri, who was banished from Britain in 2005. Bakri praised the attack and said many Muslims would consider the victim a military target.
"I used to know him. A quiet man, very shy, asking lots of questions about Islam," Bakri told Reuters in northern Lebanon. "It's incredible. When I saw that, honestly I was very surprised - standing firm, courageous, brave. Not running away."
Bakri said Adebolajo had lost contact with Al Muhajiroun in 2005. Bakri's successor as leader of the organisation, Anjem Choudary, has said Adebolajo was in contact until two years ago.
Communities Secretary Eric Pickles said there would be a thorough investigation into the role of the police and intelligence agencies. The incident underlined how "difficult it is in a free society to be able to control everyone", he added.
The attack was the first Islamist killing since July 2005, when four suicide bombers struck London transport. At that time, questions were also raised about the security services after it was revealed two of the bombers had been identified in a surveillance operation but were not followed up.
Sources familiar with the investigation have said no sign has emerged so far of direct links between the attack and an Islamist insurgency in the suspects' ethnic homeland Nigeria. Their surnames suggest they are from the Christian south of Nigeria, not the Muslim north where insurgents are active.
A Nigerian government source said there was no evidence the Woolwich suspects were linked to groups in west Africa.
The murder, just a month after the Boston Marathon bombing, revived fears of "lone wolves" who may have had no direct contact with al Qaeda but plan their own attacks. The simplicity of the attack may have made prevention particularly difficult.
Peter Clarke, the former head of London's Counter Terrorism Command who led the investigation into the 2005 bombings, said if the men did turn out to be acting alone, it showed the difficulty the security services faced in trying to stop them.
"Instead of having to dismantle an organisation, you are having to investigate and counter an ideology," he told Reuters.
The two men used a car to run down Rigby outside Woolwich Barracks in southeast London and then attacked him with a meat cleaver and knives, witnesses said.
The pair told bystanders they had killed a British soldier in revenge for wars in Muslim countries, but did not say how they had identified him. Rigby was not in uniform and was working locally as an army recruiter.
Britain has been on high alert since the killing. Adding to security concerns, fighter jets were scrambled on Friday to escort a Pakistan plane following a security threat. Two men were arrested suspicion of endangering an aircraft.
MI5 has 4,000 staff in Britain, up from up from 1,800 on the eve of the September 11, 2001 attacks on the United States. Though al Qaeda has made no successful attack on Britain since 2005, Britain has been the target of at least one credible terrorist plot every year, according to security chiefs.
Richard Barrett, former head of counter-terrorism at the Secret Intelligence Service MI6, Britain's foreign spy agency, said it would be impractical to track every person who expressed radical views in case they tipped over into violent extremism.
"To find the signals, the red flags as it were, I think is enormously hard," he told the BBC.
Additional reporting by Michael Holden; Editing by Peter Graff